High-voltage field-effect transistor and method of making the same

ABSTRACT

The high-voltage transistor device comprises a semiconductor substrate ( 1 ) with a source region ( 2 ) of a first type of electrical conductivity, a body region ( 3 ) including a channel region ( 4 ) of a second type of electrical conductivity opposite to the first type of conductivity, a drift region ( 5 ) of the first type of conductivity, and a drain region ( 6 ) of the first type of conductivity extending longitudinally in striplike fashion from the channel region ( 4 ) to the drain region ( 6 ) and laterally confined by isolation regions ( 9 ). The drift region ( 5 ) comprises a doping of the first type of conductivity and includes an additional region ( 8 ) with a net doping of the second type of conductivity to adjust the electrical properties of the drift region ( 5 ). The drift region depth and the additional region depth do not exceed the maximal depth ( 17 ) of the isolation regions ( 9 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 14/385,487 filed Sep. 15, 2014, which is the national stage ofInternational Application No. PCT/EP2013/051499, filed on Jan. 25, 2013,which claims the benefit of priority of European Patent Application No.12159949.2, filed on Mar. 16, 2012, all of which are hereby incorporatedby reference in their entirety for all purposes.

DESCRIPTION

The invention relates to a high-voltage transistor device which can beimplemented in a low-voltage CMOS technology, and a method of producingthe transistor device.

An increasing range of applications of high-voltage devices makes use ofCMOS circuits, which are designed for low voltages. Small channellengths of low-voltage transistors are achieved by means of high dopantconcentrations at the device surface, which do not allow to obtain thecondition of RESURF (REduced SURface Field) and high breakdown voltagesof the high-voltage devices. An integration of high-voltage andlow-voltage devices is desired in order to reduce the costs ofmanufacturing, but then alignment steps using additional masks, whichare not required by the low-voltage devices alone, may be necessary. Ifthe intended application of a high-voltage transistor does not requirelarge low-ohmic drivers, an integration of high- and low-voltagetransistors without any additional process lithography alignment stepsis the preferred solution.

WO 2009/050669 A2 and corresponding US 2010/0213517 A1 describe animplementation of medium/high-voltage semiconductor devices within aCMOS technology. A semiconductor region provided as a drift region isarranged between dielectric regions, which may be STI (shallow trenchisolation) regions. Electrically conductive gate extensions, which maybe striplike or tapered polysilicon fingers or field plates, arearranged above the dielectric regions and form capacitors with thesemiconductor region and the intermediate sections of the dielectricregions. The gate extensions generate a distribution of the electricalpotential which is intended to help deplete the drift region even if itsdopant concentration is higher than usual. The depletion may not sufficeif the voltage drops mainly between the gate extensions and the driftregion, owing to the high dielectric constant of the dielectric regionsand the high conductivity of the drift region.

It is an object of the present invention to provide a high-voltagetransistor device that can be manufactured within a standard process forlow-voltage devices and to disclose a method of production for thishigh-voltage transistor device.

This object is achieved with the high-voltage transistor deviceaccording to claim 1 and with the method of producing the high-voltagetransistor device according to claim 10. Embodiments and variants derivefrom the dependent claims.

The high-voltage transistor device comprises a semiconductor substrate,a source region and a drain region of a first type of electricalconductivity arranged in the substrate at a distance from one another, achannel region of a second type of electrical conductivity, which isopposite to the first type of electrical conductivity, arranged betweenthe source region and the drain region, a striplike drift regionlongitudinally extending from the channel region to the drain region,and isolation regions laterally confining the drift region transverse toits longitudinal extension. The drift region comprises a doping of thefirst type of conductivity and includes an adjustment region with a netdoping of the second type of conductivity. The drift region and theadjustment region do not exceed the maximal depth of the isolationregions.

In an embodiment of the high-voltage transistor device the adjustmentregion is tapered to have different lateral widths in such a way thatthe lateral widths of the adjustment region decrease in the directionfrom the channel region towards the drain region.

In a further embodiment the isolation regions are shallow trenchisolations. The isolation regions may instead be formed by other means,especially as field oxide regions.

In further embodiments the isolation regions are spaced apart by adistance in the range between 0.3 μm and 0.7 μm or more specifically inthe smaller range between 0.4 μm and 0.6 μm. This distance may be equalto a maximal lateral width of the adjustment region.

In a further embodiment a drain contact region of the first type ofconductivity is arranged at the drain region, the drift region abuttingthe drain contact region, and the adjustment region is arranged at adistance from the drain contact region.

In further embodiments a plurality of drift regions of the first type ofconductivity are arranged parallel to one another and separated byisolation regions, the drift regions comprising a doping of the firsttype of conductivity and including adjustment regions with a net dopingof the second type of conductivity. The drift regions and the adjustmentregions do not exceed a maximal depth of the isolation regions.

In a further embodiment the adjustment regions are tapered to havedifferent lateral widths in such a way that the lateral widths of theadjustment regions decrease in the direction from the channel regiontowards the drain region.

In a method of producing the high-voltage transistor device, isolationregions confining a striplike area are formed in a semiconductorsubstrate, a drain region of a first type of electrical conductivity anda body region of a second type of electrical conductivity, which isopposite to the first type of conductivity, are formed at opposite endsof the striplike area, and a gate electrode is formed above the bodyregion. A dopant for the first type of conductivity is implanted in thestriplike area in such a way that the dopant does not exceed the maximaldepth of the isolation regions. A source region of the first type ofconductivity is formed in the body region. A dopant for the second typeof conductivity is implanted in the striplike area in such a way thatthe dopant does not exceed the maximal depth of the isolation regionsand a net doping of the second type of conductivity is obtained in anadjustment region.

In a variant of the method the adjustment region is formed using aparticular mask to implant the dopant for the second type ofconductivity in the striplike area. The opening of the mask is limitedby an oblique edge, which runs across the striplike area neither inparallel nor perpendicularly to the longitudinal extension of thestriplike area.

In a further variant of the method the adjustment region is formed usinga mask that completely covers the gate electrode.

In a further variant of the method the implantations of the dopants forthe first and second types of conductivity in the striplike area betweenthe isolation regions are performed in a way that is self-aligned withrespect to the gate electrode.

In further variants of the method a plurality of isolation regions arearranged parallel to one another confining a plurality of parallelstriplike areas. A dopant for the first type of conductivity isimplanted in the striplike areas to a maximal depth that does not exceedthe maximal depth of the isolation regions, and a dopant for the secondtype of conductivity is implanted in the striplike areas to a maximaldepth that does not exceed the maximal depth of the isolation regions insuch a way that a net doping of the second type of conductivity isobtained in adjustment regions.

In a further variant of the method the implantation of the dopant forthe second type of conductivity in the striplike areas is performed bymeans of a mask having a plurality of parallel edges each traversing oneof the striplike areas in a direction that is oblique to a longitudinalextension of the striplike area.

The following is a further explanation of the invention and itsadvantages by a detailed description of exemplary embodiments inconjunction with the accompanying drawings.

FIG. 1 shows a perspective cross-section of an embodiment of thetransistor device.

FIG. 2 shows a section of a top view encompassing the area of a driftregion.

FIG. 3 shows a cross-section according to FIG. 1 for a state ofproduction.

FIG. 4 shows a cross-section according to FIG. 3 for a further state ofproduction.

FIG. 5 shows the cross-section of FIG. 4 with the isolation regionsrepresented as if being transparent.

FIG. 6 shows a section of a top view of the transistor device coveredwith a periodic mask.

FIG. 7 shows a section of a top view according to FIG. 6 with a periodicmask of a different shape.

FIG. 1 shows a high-voltage transistor device in a perspectivecross-section. The device comprises a semiconductor substrate 1, whichmay be silicon, for example, with doped regions, which are arranged ator near a main surface of the substrate 1. The doped regions include asource region 2, a body region 3 with a channel region 4 formed at asurface area of the body region 3, and a striplike drift region 5extending from the channel region 4 to a drain region 6. The sourceregion 2, the drift region 5, and the drain region 6 have a first typeof electrical conductivity. A drain contact region 7 for an externalelectrical contact may be provided at the surface of the substrate 1adjacent to the drift region 5, as shown in FIG. 1, or may be arrangedat a distance from the drift region 5 on the drain region 6. The draincontact region 7 has the first type of electrical conductivity andpreferably a higher doping concentration than the drain region 6. Thehigher doping concentration in the drain contact region 7 may especiallybe produced by an implantation of dopants for the drift region 5 if thisimplantation is simultaneously performed in the drain region 6.Additionally or instead there may be a dedicated implantation of highdose for the first type of electrical conductivity particularly for theformation of the drain contact region 7, either adjacent to the driftregion 5 or at a distance from the drift region 5. The body region 3 isdoped to have a second type of electrical conductivity, which isopposite to the first type of conductivity. The body region 3 and thedrain region 6 can be produced as doped wells by an implantation ofdopants for opposite types of conductivity. In a high-voltage NMOStransistor the first type of conductivity is n-type conductivity and thesecond type of conductivity is p-type conductivity. In a high-voltagePMOS transistor the first type of conductivity is p-type conductivityand the second type of conductivity is n-type conductivity.

The drift region 5 is formed by a strip of semiconductor material in theshape of a fin or bar and is laterally confined by isolation regions 9,which are arranged at a distance from one another. The lateral width ofthe drift region 5 is thus defined by the distance between the isolationregions 9, which may be essentially constant along the longitudinalextension of the drift region 5. In FIG. 1 the isolation regions 9 arerepresented as if they were transparent in order to show the shape andposition of the drift region 5. An upper surface of the drift region 5may be a striplike area 15 of essentially constant width. This striplikearea 15 may be covered by a thin dielectric layer, especially by anoxide of the semiconductor material, which may also be used as a gatedielectric insulating the gate electrode from the semiconductormaterial. The isolation regions 9 may be formed by shallow trenchisolations (STI), especially with an oxide of the semiconductormaterial. The vertical dimension or depth of the drift region 5 does notexceed the maximal depth 17 of the isolation regions 9. The drift region5 is thus limited on both lateral sides completely by the boundaries ofthe isolation regions 9.

The electrical properties of the drift region 5 are modified andadjusted by an adjustment region 8 with a net doping of the second typeof electrical conductivity, which is included in the drift region 5 anddoes not exceed the depth 17 of the isolation regions 9 either. Themaximal depth of the adjustment region 8 may be less than the depth ofthe drift region 5, so that a net doping of the first type ofconductivity is present in a lower layer portion of the drift region 5,as indicated in FIG. 1. By means of the adjustment region 8 a gradedelectrical conductivity can be achieved within the drift region 5.

FIG. 2 shows a section of a plan view of the device, encompassing thearea of the striplike drift region 5 extending longitudinally from thechannel region 4, which abuts one end of the drift region 5, to thedrain region 6 or the drain contact region 7 at the opposite end. Thedrift region 5 is limited on both lateral sides by the isolation regions9, so that the surface of the drift region 5 is a striplike area 15. Thedirection of longitudinal extension 20 is indicated in FIG. 2 with anarrow. The distance 19 between the isolation regions 9 may be constantalong the drift region 5.

The adjustment region 8 is tapered in the embodiment according to FIGS.1 and 2. The lateral width 18, 18′ of the adjustment region 8 decreasesin the direction from the channel region 4 towards the drain region 6.The areas of the cross-sections of the adjustment region 8 which areperpendicular to the longitudinal extension 20 of the drift region 5thus decrease with increasing distance from the channel region 4 anddecreasing distance from the drain region 6.

The maximal lateral width 18 of the adjustment region 8 may be equal tothe distance 19 between the isolation regions 9 and hence equal to thelateral width of the drift region 5, as in the example shown in FIG. 2,or the maximal lateral width 18 of the adjustment region 8 may besmaller than the distance 19 between the isolation regions 9. Theadjustment region 8 may be arranged at a distance from the channelregion 4 or at a distance from the drain region 6 or, as in theembodiment according to FIGS. 1 and 2, at distances both from thechannel region 4 and from the drain region 6.

FIG. 3 shows a perspective cross-section according to FIG. 1 for aprocess state of the method of producing the transistor device. At amain surface of a semiconductor substrate 1, the body region 3 and thedrain region 6 are formed, preferably as doped wells of opposite typesof conductivity by implantations of different dopants. Between the bodyregion 3 and the drain region 6 the isolation regions 9 are arranged inparallel at a distance from one another.

The isolation regions 9 may be formed as shallow trench isolations byetching shallow trenches, which are subsequently filled with aninsulating or dielectric material, particularly with an oxide of thesemiconductor material. Shallow trench isolations are preferablyproduced before the implantations of the doped wells. In embodiments ofthe device the distance 19 between the isolation regions 9 may lie inthe range between 0.3 μm and 0.7 μm or may be restricted to the rangebetween 0.4 μm and 0.6 μm. The distance 19 may be typically 0.5 μm, forexample.

FIG. 3 shows a thin cover layer 13 on the substrate 1 between theisolation regions 9. The cover layer 13 may comprise the same materialas the isolation regions 9, especially an oxide of the semiconductormaterial. The boundaries between the isolation regions 9 and thesubstrate region that is provided for the drift region 5 are indicatedin FIG. 3 with broken lines as hidden contours under the cover layer 13.The material of the cover layer 13 may also be provided as a gatedielectric 14 to insulate the gate electrode 10, which is arranged abovethe channel region 4, from the semiconductor material. The gateelectrode 10 may be polysilicon, for instance.

A first mask 11, which may be a resist, is applied for a subsequentimplantation of a dopant for the first type of conductivity in the driftregion 5, as indicated by the arrows in FIG. 3. The implantation ispreferably performed in a way which is self-aligned with respect to thegate electrode 10. The implantation depth does not exceed the depth 17of the isolation regions 9.

FIG. 4 shows a perspective cross-section according to FIG. 3 for afurther process state after the first mask 11 has been substituted witha second mask 12, which may be a resist as well. The lateral edge 16limiting the opening of the second mask 12 is arranged obliquely withrespect to the longitudinal extension 20 of the drift region 5, whichmeans that the edge 16 runs neither in parallel nor perpendicularly tothe longitudinal extension 20 of the drift region 5. An implantation ofa dopant for the second type of conductivity is then performed in thedrift region 5, as indicated by the arrows in FIG. 4. This implantationmay also be performed in a way which is self-aligned with respect to thegate electrode 10. The implantation depth does not exceed the depth 17of the isolation regions 9 and may be shallower than the previousimplantation of the dopant for the first type of conductivity. Theimplantation of the dopant for the second type of conductivity isprovided to produce the adjustment region 8 within the drift region 5,according to the shape of the second mask 12.

If the oblique edge 16 of the second mask 12 traverses the total widthof the striplike area 15 between the isolation regions 9, the precisionof the alignment of the second mask 12 is not critical, and a possiblemisalignment of the second mask 12 can be taken account of by a suitabledefinition of the length of the drift region 5. The implantationsprovided for the drift region 5 and for the adjustment region 8 render anet doping concentration in the drift region 5 which allows dropping thedrain/source potential in a controlled way.

FIG. 5 shows the perspective cross-section according to FIG. 4 with theisolation regions 9 and the cover layer 13 represented as if they weretransparent, similar to FIG. 1. FIG. 5 shows the shape of the driftregion 5 as a fin or bar and the tapering section of the striplike area15 of the upper surface of the drift region 5 that is not covered by thesecond mask 12. The doping of the drift region 5 that is obtained by thepreceding implantation is counterdoped at least in a section of theuncovered area in order to form the adjustment region 8 with a netdoping of the second type of conductivity. The use of a second mask 12having an opening that is limited by an edge 16 arranged obliquely withrespect to the drain region 5 results in a tapering shape of theadjustment region 8 according to the embodiment shown in FIGS. 1 and 2.

If the high-voltage transistor device comprises a high-voltage NMOStransistor, for example, and the implantations of donor atoms for thedrift region 5 and acceptor atoms for the adjustment region 8 havenearly the same depth and thus comparable profiles, a suitable netconductivity in the drift region 5 may be obtained by an effective dose,which is the difference between the dose of donor atoms and the dose ofacceptor atoms, of more than 1·112 cm-2. If the implantation of acceptoratoms for the adjustment region 8 is shallower than the implantation ofdonor atoms, the donor concentration remaining below the adjustmentregion 8 should be equivalent to a dose of more than 1·112 cm-2.

A device having a larger width may comprise a plurality of striplikedrift regions 5 arranged in parallel and separated by isolation regions9 from one another. The drift regions 5 are provided with adjustmentregions 8 as described above. In order to produce the adjustment regions8, the dopant for the second type of conductivity may be implanted inthe striplike areas 15 by means of a second mask 12 that has a pluralityof parallel openings. Examples are shown in FIGS. 6 and 7.

FIG. 6 shows a section of a top view of the transistor device coveredwith the second mask 12 above a section of the substrate 1 that isoutlined with broken lines. The isolation regions 9 are arranged in thesubstrate 1 along essentially the same direction at a distance from oneanother and separated by parallel striplike regions that are provided asdrift regions 5. The striplike areas 15 of the drift regions 5,indicated in FIG. 6 by braces, may laterally be limited by parallelboundaries if the isolation regions 9 have constant distances 19 fromone another. The position of the gate electrode 10 with respect to theopenings of the second mask 12 is also indicated in FIG. 6. The openingsof the second mask 12 have boundaries which are oblique with respect tothe longitudinal extension 20 of the drift region 5 at least above thestriplike areas 15. The oblique edges 16 of the second mask 12 thusrender tapered adjustment regions 8 as described above. The sequence ofthe oblique edges 16 that traverse the striplike areas 15 is preferablyperiodic, so that the oblique edges 16 are parallel to one another andarranged at equal distances. In the example of FIG. 6, the openings ofthe second mask 12 leave sections of the edge of the gate electrode 10uncovered, so that the implantation of the dopant for the adjustmentregions 8 is self-aligned with respect to the gate electrode 10.

FIG. 7 shows a section of a top view according to FIG. 6 for anotherembodiment of the second mask 12, which completely covers the gateelectrode 10. In the embodiment according to FIG. 7 the implantation ofthe dopant for the adjustment regions 8 is not self-aligned with respectto the gate electrode 10, but the dopant is implanted at a distance fromthe gate electrode 10. This may be favorable for some embodiments,depending on the lateral diffusion of the implanted dopants, especiallyif the implantation dose is high and/or the implantation angle is steep.The example of FIG. 7 may be preferred in these cases to secure that thearrangement of the adjustment regions 8 does not prevent the chargecarriers from passing the drift region.

1. A method of producing a high-voltage transistor device, comprising:forming isolation regions in a semiconductor substrate, the isolationregions confining a striplike area; forming a drain region of a firsttype of electrical conductivity and a body region of a second type ofelectrical conductivity, which is opposite to the first type ofconductivity, at opposite ends of the striplike area; forming a gateelectrode above the body region; implanting a dopant for the first typeof conductivity in the striplike area; forming a source region of thefirst type of conductivity in the body region; the implantation beingperformed in such a way that the dopant for the first type ofconductivity does not exceed a maximal depth of the isolation regions;and a dopant for the second type of conductivity being implanted in thestriplike area in such a way that the dopant does not exceed the maximaldepth of the isolation regions and a net doping of the second type ofconductivity is obtained in an adjustment region.
 2. The method of claim1, wherein the adjustment region is formed using a mask having anopening that is limited by an edge arranged obliquely with respect to alongitudinal extension of the striplike area to implant the dopant forthe second type of conductivity in the striplike area.
 3. The method ofclaim 1 or 2, wherein the adjustment region is formed using a mask thatcompletely covers the gate electrode.
 4. The method of claim 1 or 2,wherein the implantations of the dopants for the first and second typesof conductivity in the striplike area between the isolation regions areperformed in a way that is self-aligned with respect to the gateelectrode.
 5. The method of claim 1 or 2, wherein a plurality ofisolation regions are arranged parallel to one another confining aplurality of parallel striplike areas; a dopant for the first type ofconductivity is implanted in the striplike areas to a maximal depth thatdoes not exceed the maximal depth of the isolation regions; and a dopantfor the second type of conductivity is implanted in the striplike areasto a maximal depth that does not exceed the maximal depth of theisolation regions in such a way that a net doping of the second type ofconductivity is obtained in adjustment regions.
 6. The method of claim5, wherein the implantation of the dopant for the second type ofconductivity in the striplike areas is performed by means of a maskhaving a plurality of parallel edges each traversing one of thestriplike areas in a direction that is oblique to a longitudinalextension of the striplike area.
 7. A method of producing a high-voltagetransistor device, comprising: forming isolation regions in asemiconductor substrate, the isolation regions confining a striplikearea; forming a drain region of a first type of electrical conductivityand a body region of a second type of electrical conductivity, which isopposite to the first type of conductivity, at opposite ends of thestriplike area; forming a gate electrode above the body region;implanting a dopant for the first type of conductivity in the striplikearea; forming a source region of the first type of conductivity in thebody region; the implantation being performed in such a way that thedopant for the first type of conductivity does not exceed a maximaldepth of the isolation regions; a dopant for the second type ofconductivity being implanted in the striplike area in such a way thatthe dopant does not exceed the maximal depth of the isolation regionsand a net doping of the second type of conductivity is obtained in anadjustment region; and the adjustment region being produced having atapered shape with different lateral widths in such a way that theselateral widths decrease in the direction from the channel region towardsthe drain region.
 8. A method of producing a high-voltage transistordevice, comprising: forming isolation regions in a semiconductorsubstrate, the isolation regions confining a striplike area; forming adrain region of a first type of electrical conductivity and a bodyregion of a second type of electrical conductivity, which is opposite tothe first type of conductivity, at opposite ends of the striplike area;forming a gate electrode above the body region; implanting a dopant forthe first type of conductivity in the striplike area; forming a sourceregion of the first type of conductivity in the body region; theimplantation being performed in such a way that the dopant for the firsttype of conductivity does not exceed a maximal depth of the isolationregions; a dopant for the second type of conductivity being implanted inthe striplike area in such a way that the dopant does not exceed themaximal depth of the isolation regions and a net doping of the secondtype of conductivity is obtained in an adjustment region; the driftregion being formed abutting a drain contact region; and the adjustmentregion being arranged at a distance from the drain contact region. 9.The method of claim 7 or 8, wherein the adjustment region is formedusing a mask having an opening that is limited by an edge arrangedobliquely with respect to a longitudinal extension of the striplike areato implant the dopant for the second type of conductivity in thestriplike area.
 10. The method of claim 7 or 8, wherein the adjustmentregion is formed using a mask that completely covers the gate electrode.11. The method of claim 7 or 8, wherein the implantations of the dopantsfor the first and second types of conductivity in the striplike areabetween the isolation regions are performed in a way that isself-aligned with respect to the gate electrode.
 12. The method of claim7 or 8, wherein a plurality of isolation regions are arranged parallelto one another confining a plurality of parallel striplike areas; adopant for the first type of conductivity is implanted in the striplikeareas to a maximal depth that does not exceed the maximal depth of theisolation regions; and a dopant for the second type of conductivity isimplanted in the striplike areas to a maximal depth that does not exceedthe maximal depth of the isolation regions in such a way that a netdoping of the second type of conductivity is obtained in adjustmentregions.
 13. The method of claim 12, wherein the implantation of thedopant for the second type of conductivity in the striplike areas isperformed by means of a mask having a plurality of parallel edges eachtraversing one of the striplike areas in a direction that is oblique toa longitudinal extension of the striplike area.